1. Field of the Invention
This invention generally relates to a lamp circuit in which an AC power source of constant current type, i.e. a constant current regulator (hereinafter designated as CCR), is connected to a plurality of lamps through a plurality of isolation transformers, respectively, and more particularly to a lamp circuit with a disconnected lamp detecting device in which the number of disconnected lamps is detected by means of a change of the voltage-time integral which depends on the magnetic saturation of the isolation transformers in proportion to the number of the disconnected lamps.
2. Description of the Prior Art
A conventional thyristor type CCR, as shown in FIG. 1, has been employed as a power supply for a lamp circuit for use on a landing strip or runway lighting in an airport.
In FIG. 1, numeral 1 designates an AC power source, 2 designates a smoothing reactor, 3 and 4 designates thyristors, 5 designates a power transformers, or output transformer, 6 designates a current transformer, 7 designates a differential amplifier, 8 designates a gate controlling circuit, 9 designates a potential transformer, 10 designates a disconnected lamp detecting circuit, 11 designates an alarm circuit, and 13 designates a reference current input adjuster. Reference numeral 12 designates a series lamp circuit which comprises a plurality of series connected isolation transformers 121, the primary windings of which are connected in series. The secondary winding of each transformer is connected to an electric lamp 122.
As shown in FIG. 1, the output current of the thyristor type CCR is detected by the current transformer 6 and is compared with the signal Cs of the reference current input adjuster 13 in the differential amplifier 7. The differential amplifier 7 amplifies the compared signal and produces a signal Go.
The gate signals G.sub.1 and G.sub.2 of the gate controlling circuit 8 are supplied to the respective gates of the thyristors 3 and 4 so as to maintain the output current of the CCR at a constant level, i.e. to keep the intensity or brilliance of the lamps at a constant level.
The disconnected lamp detecting circuit 10 is shown in FIG. 2 in detail. After the voltage signal v of the potential transformer 9 and the current signal i of the current transformer 6 are rectified by respective full-wave rectifiers D.sub.1 and D.sub.2, the difference signal e between the two outputs of the rectifiers D.sub.1 and D.sub.2 is produced. After smoothing the difference signal e, the smoothed signal is supplied to the base terminal of a transistor Tr which produces a signal A to activate the alarm circuit 11 when the value of the smoothed signal exceeds a predetermined value. The alarm circuit 11 indicates the alarm condition by means of a buzzer or a lamp in response to the alarm signal A.
In the case where no lamp is disconnected, the voltage signal v and the current signal i become respectively waveforms v.sub.1 and i.sub.1, as shown in FIGS. 3(a) and 3(b). Therefore, the difference signal e between these signals becomes the waveform e.sub.1 shown in FIG. 3(c). At this time since the transistor Tr is not turned on, the alarm signal A is not produced.
If it is assumed that a number of the lamps 122 are disconnected, the voltage signal v and the current signal i become respectively the waveforms v.sub.2 and i.sub.2 shown in FIGS. 3(d) and 3(e). Therefore, the waveform of the difference signal e.sub.2 is shown in FIG. 3(f). The smoothed difference signal e.sub.2 makes the transistor Tr operate thereby producing the alarming signal.
The detection of the disconnected lamps is thus carried out. However, the waveforms of the voltage signal v and the current signal i are often deformed by disturbances such as noise from the analog signals. Therefore, even though a lamp is not actually disconnected, the voltage value, from which the difference e of the waveform is smoothed, becomes or reaches a value sufficient to operate the transistor Tr of the disconnected lamp detecting circuit 10. As a result, a false alarm signal is produced.
To prevent such an above-mentioned misdetection, the operating voltage value, which makes the transistor Tr operate, must be set to a larger value than the previously set value. Therefore it is impossible to detect a disconnecting lamp with high-sensitivity. Furthermore, the sensitivity of the detection is within the limits of about ten percent of the rated load, and thus the desired sensitivity of detection within a limit of about five percent of the rated load cannot be achieved.
There is the danger of increasing the risks to aircraft due to a defect of the landing or runway lighting in an airport. Moreover, when an isolation transformer, in which the secondary winding has been opened by a disconnected lamp, is left for a long period of time, there is a danger of a winding short upon the application of a high-voltage pulse and the danger of a burn-out due to rising temperature. Furthermore, to display the number of actually disconnected lamps in addition to the alarm function, it is necessary to provide a new display circuit.